Property allowance is deducted on top of expenses already netted from property income #1900

property_allowance_deduction returns min_(property_income, property_allowance) and taxable_property_income subtracts it from property_income. That input is net of expenses. The enhanced FRS imputes it from SPI INCPROP, which the SPI Public Use Tape documentation (UKDS SN 9422, 2022-23) defines as "Net income from UK and overseas property (income - expenses)". So the allowance is stacked on expenses already deducted.

This is the same defect #1880 fixed for the trading allowance (PR #1881).

Statute. The property allowance is ITTOIA 2005 Part 6A Chapter 2 (ss. 783B–783BQ):

• s. 783BD: the allowance is £1,000.
• s. 783BF, full relief: relievable receipts and associated expenses are not brought into account.
• s. 783BH, partial relief (by election under s. 783BK): receipts are brought in, "No relevant expenses are brought into account", and the deductible amount is brought in instead.
• s. 783BI(4): partial relief cannot create a loss.
• s. 783BB: rent-a-room receipts are excluded.
• s. 783BL: no relief where the s. 274A finance-cost reducer applies.

Other defects in the same code.

• The variable cites s. 783AF, which is a trading-allowance section. It should cite ss. 783BF, 783BH and 783BD.
• parameters/gov/hmrc/income_tax/allowances/property_allowance.yaml starts at 2005-04-01, but the allowance has effect from 2017-18 (F(No.2)A 2017 Sch 3 para 13).

Suggested fix. Mirror #1881:

• Add an optional gross property receipts input. With receipts known, relief is max(0, min(net, 1000 − expenses)).
• With receipts unknown, give full relief only where net property income is within £1,000.
• Add YAML tests and Hypothesis property tests: bounds, a statutory reference, no stacking, and monotonicity.
• Measure the revenue effect on the published enhanced FRS.